Semiconductor wafer backside contamination is created during many different semiconductor integrated circuit manufacturing steps. In many cases, coming into contact with the wafer handling/processing equipment contaminates the wafer backside. For example, robotic components such as end effectors, wafer chucks, and wafer cassettes and other wafer storage devices cause different kinds of particles to be firmly attached to the backside of a wafer. This may occur during a variety of wafer exchange procedures and is associated with the friction, high vacuum contact forces and electrostatic charge buildup and discharge cycles that occur during such transfers.
Other types of contamination on a wafer backside may result from various chemical processes to which the wafer is subjected during processing. Although these processes are primarily intended to affect the top surface of the wafer, inevitably some effect on the wafer backside may result as well. For example, residues of photo resist polymer may adhere to the wafer backside during lithographic procedures (e.g., coating, baking and stripping). Metal contamination of the backside may also occur during various types of dry or wet processes.
Contamination of the wafer backside leads to overall yield degradation as the backside contamination may ultimately reach the wafer's top surface, harming sensitive devices. In addition, backside contamination that affects a wafer's planarity may cause problems with photolithographic procedures. For example, large particles adhering to the backside may act like bumps and when ultra flat chucks are used to support the wafer during photolithographic procedures, these bumps may cause the top side of the wafer to be positioned out of focus for the lithographic procedure, again resulting in lower overall component yield.
Others have recognized the problems that result from wafer backside contamination during wafer processing and have proposed schemes for cleaning these backside surfaces. For example, some have advocated post-CMP cleaning of the backside by brush scrubber cleaning, megasonic cleaning or even wet etching. Wet etching has also been proposed for removing films from wafer backsides. However, to date none of these techniques have been combined with methods for wafer backside inspection so as to know whether or not the cleaning has been successful.
Where wafer backside inspection has been employed it is generally seen as a time consuming task, requiring the use of dedicated machinery to grasp the wafer by its edge and rotate it so that conventional topside inspection tools can be used to examine the backside. That is, the backside inspection involves physically contacting the wafer so as to invert it to such a position that metrology tools otherwise used for inspecting the top surface of a wafer may now be used to inspect the backside thereof. Such a solution is less than optimum for several reasons, including the fact that physically manipulating the wafer in this fashion may itself cause damage, leading to still further yield reductions.
Thus, what is needed is a new method and apparatus for wafer backside cleaning and inspection.